Modern furnaces, such as those used in sheet glass forming, typically include multiple sections or "zones" wherein heated combustion products from a plurality of discrete gas burner assemblies are used to carefully maintain desired temperatures throughout these zones while further responding to variations in attendant heat loads. For example, the passage of a workpiece, such as a glass sheet, through these zones induces temperature variations within each zone which must be corrected in real time in order to achieve the desired heating of the workpiece.
More specifically, when a relatively large heat input must be applied to the furnace and its load, in order to maintain the setpoint temperature of the furnace and to ensure that the desired rate of heat transfer to the load is achieved, the output of the burner systems will increase in response to the thermal loading. The output of the gas burners may increase to 100% of the burner's rate capacity which is referred to as "high-fire". At a later stage in the process, as the load begins to approach the set point temperature, the heat input applied by the burner must be lowered to prevent overheating. The output of the burner systems will then decrease in response to the thermal loading. The output of the burners may decrease to 10% (or less) of the burner's rated capacity which is referred to as "low-fire". The ratio of the maximum to the minimum thermal output of a burner is referred to as the turndown ability of the system. Modern furnace construction provides for minimal heat loss at operating temperatures, with an attendant requirement of relatively high turndown ratios of at least 10 to 1.
Two common ways of achieving turndown are thermal turndown and stoichiometric turndown. During thermal or "excess air" turndown, the flow of fuel is reduced while the air flow is held constant, effectively lowering the fuel-to-air ratio. Because the excess combustion air is heated by combustion, the released heat is diluted and the temperature of the combustion products exiting the burner's combustion chamber is effectively reduced.
Under the preferred approach, thermal turndown is achieved by reducing both combustion air and fuel so as to simultaneously increase the amount of excess combustion air in relation to the fuel. At the high-fire rate, the level of excess air is 10% which is very close to stoichiometric. At the low-fire rate, the level of excess air is 1000%. Under the preferred approach, the turndown of fuel is 28:1. The 1000% excess air reduces the hot mix temperature of combustion products which further reduces the thermal output of the burner. The effective thermal turndown then becomes 100:1 or greater which yields excellent control of furnace temperatures for all loading conditions.
Under one prior art burner capable of high thermal turndown, a constant-speed blower is used to supply combustion air to the combustion chamber of a given burner assembly. A butterfly valve located between the blower discharge and the combustion chamber is adjusted from a minimum flow, "low-fire" position (typically an almost closed position) through a maximum flow, "high-fire" position (often perhaps an 85 degree position due to flow nonlinearity through the butterfly valve) to thereby modulate the quantity of combustion air supplied to the combustion chamber in response to a demand signal. A proportional pressure regulator responsive to the static pressure in the burner assembly downstream of the butterfly valve meters the flow of fuel into the burner's combustor, whereby an appropriate air-fuel ratio is achieved within the combustion chamber for any given mass flow rate of combustion air between low-fire and high-fire conditions.
The butterfly valve of such prior art burners is typically set to a nearly closed position for low-fire to allow only the low-fire mass flow of air to enter the burner's combustion chamber. The valve may also be set to a fully closed position for low-fire with an appropriately sized bypass around the valve to allow only the low-fire mass flow of air to enter the burner's combustion chamber. The valve is typically driven itself from this low-fire position to its high-fire position using a dedicated stepper motor via a flexible coupling or linkage arrangement. In one prior art burner assembly, a sixteen-position stepper motor is employed, whereby the valve plate is driven between its low-fire and high-fire positions in equal increments of approximately 5 degrees of rotation per step. Adjustable limit switches are often used to indicate when the "low-fire" and "high-fire" valve plate positions are achieved.
The lost motion characteristic of such mechanical linkages and the relatively limited resolution of the stepper motor combine with the substantial nonlinear relationship between relative position of the valve plate and the corresponding mass flow rate of air through the butterfly valve to provide relatively limited control of the heat output of the burner assembly, with the further likelihood that the burner assembly will undesirably "hunt" between an upper and lower level of heat output, with a resulting furnace temperature variance of perhaps 5.degree. F. or greater.
Prior art gas burner assemblies are disclosed by U.S. Pat. Nos. 5,406,840 Boucher and 5,685,707 Ramsdell et al.